Method and device for retransmission

ABSTRACT

Embodiments related to retransmission in a communication system are described and depicted. In one embodiment, a retransmission entity repeats a transmission of a data transfer unit by the device after a predetermined number of other transmitted data transfer units has been transmitted. The retransmission entity may also determine whether a measure for a time period since the first transmission of the data transfer unit by the device has exceeded a predetermined threshold and to provide a final transmission of the data transfer unit based on the determining that the measure for the time period has exceeded the predetermined threshold.

REFERENCE TO RELATED APPLICATIONS

This application claims priority benefit of Provisional Applications61/243,190 and 61/243,191, which were both filed on Sep. 17, 2009. Theentire contents of the Provisional Applications 61/243,190 and61/243,191 are hereby incorporated by reference herein.

BACKGROUND

Modern data communication systems such as DSL communication systemstransmit a plurality of different data types. Data of high-qualityservices such as IPTV services or video services require an efficientnoise protection since missing data often provide strong disturbances ofthese services. Present impulse noise protection with Reed Solomoncoding and interleaving provide not sufficient protection for thesehigh-quality services.

Retransmission schemes have been introduced to address noise protectionfor high-quality services as well as other services. In retransmission,data transmitted over a communication link such as a subscriber line isstored at the transmitter site for some time. In case the receiver sitereceives corrupt data, for example when an impulse noise occurs, thetransmitter site retransmits the data based on a request from thereceiver again over the communication link. In order to provide highsecurity for the transmission, an implemented retransmission schemeshould be reliable and secure for situations occurring during theoperation of the system.

SUMMARY

According to an embodiment, a method includes repeating a transmissionof a previously transmitted data transfer unit by a device after apredetermined number of other data transfer units has been transmittedby the device. A determining is made whether a measure for a time periodsince the first transmission of the data transfer unit by the device hasexceeded a predetermined threshold. A final transmission of thepreviously transmitted data transfer unit is provided based on adetermining that the measure for the time period has exceeded thepredetermined threshold.

According to an embodiment, a method includes determining a first valueindicating a length of repetitive impulse noise events occurring on acommunication system when transmitting data from a first device to asecond device and determining a second value indicating a period lengthof the repetitive impulse noise events occurring on the communicationsystem. A parameter of a retransmission for the communication systemsuch as a parameter used to generate a retransmission return message isconfigured based on the first and second value.

According to a further embodiment, a method includes a determining for adata transfer unit in a retransmission queue of a device whether ameasure for a time period since the first transmission of the datatransfer unit by the device exceeds a predetermined limit. If it isdetermined for the data transfer unit that the measure for a time periodsince the first transmission exceeds a predetermined limit, a finaltransmission of the data transfer unit is provided.

According to a further embodiment, a method of operating a communicationsystem includes transferring a plurality of information related to aretransmission return message from a second device to a first device andproviding by the second device in a first mode for at least one firstinformation of the plurality of information a predefined value. Thesecond device provides in a further mode for the at least one firstinformation a value which is derived from at least one data transferunit received by the second device from the first device.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 shows a block diagram according to an embodiment.

FIG. 2 shows a structure of a data transfer unit according to anembodiment.

FIG. 3 shows a block diagram according to an embodiment.

FIG. 4 shows a flow diagram according to an embodiment.

FIG. 5 shows an exemplary data transfer unit receive sequence.

FIG. 6a and FIG. 6b show exemplary receive and transmit sequences.

FIG. 7 shows an exemplary flow diagram.

FIG. 8a-d show exemplary data transfer sequences.

FIG. 9 shows an exemplary process flow diagram.

DETAILED DESCRIPTION

The following detailed description explains exemplary embodiments of thepresent invention. The description is not to be taken in a limitingsense, but is made only for the purpose of illustrating the generalprinciples of embodiments of the invention while the scope of protectionis only determined by the appended claims.

In the exemplary embodiments shown in the drawings and described below,any direct connection or coupling between functional blocks, devices,components or other physical or functional units shown in the drawingsor described herein can also be implemented by an indirect connection orcoupling. Functional blocks may be implemented in hardware, firmware,software, or a combination thereof.

Further, it is to be understood that the features of the variousexemplary embodiments described herein may be combined with each other,unless specifically noted otherwise.

In the various figures, identical or similar entities, modules, devicesetc. may have assigned the same reference number.

The embodiments described below with respect to the figures are directedto provide a new concept for retransmission and configuration ofretransmission in communication systems. FIG. 1 shows an embodiment of acommunication system 100 having a communication device 102 and acommunication device 104. The communication system 100 may for examplebe a DSL system, in which the first and second devices are connected viaa twisted pair copper line. The communication system 100 establishes afirst communication channel 106 from the device 102 to the device 104and a second communication channel 108 from the device 104 to the device102. The concept of communication channels is known to a person skilledin the art. In DSL for example, the first and second communicationchannel can be established on a single twisted pair copper line by usingsubcarriers of different frequency bands provided for a DMT (discretemultitone) modulation.

The device 102 has a first retransmission entity 102 a which isconfigured to provide retransmission functionality for data send via thecommunication channel 106 to the second device 104. The device 104 has asecond retransmission entity 104 a which for example provides a checkingof correct receiving of received data and generating of retransmissionreturn messages. Retransmission is a concept to provide protection ofdata against noise occurring on the communication channel. Datatransmitted over the communication channel 106 are stored in aretransmission buffer at the first device 102 and are therefore eligiblefor retransmission in case noise occurred on the line during thetransmission of the data. Thus, by using retransmission, loss of datacan be avoided when the second device 104 receives data corrupted due tonoise during the transmission. Corruption of the data during thetransmission may either result from a single event such as a Single HighImpulse Noise Event (SHINE) or from repetitive noise such as RepetitiveElectrical Impulse Noise (REIN) which is coupled in from nearbyelectrical AC-Power transmission or AC-power sources. For example, forDSL communication systems, Single High Impulse Noise Event andRepetitive Electrical Impulse Noise are one of the major noise sources.Depending on the frequency used for transmitting DC power, RepetitiveElectrical Impulse Noise is for example occurring at a frequency of 100Hz or at a frequency of 120 Hz.

A Single High Impulse Noise Event arises typically as a non-periodicstream of impulses with long duration time, high impulse power andrandom occurrence times. Such impulses may have a length up to multiplemilliseconds.

In the described embodiment of FIG. 1, only data transmitted over thefirst communication channel 106 are protected by retransmission. Inother words, only data transmitted in the direction from the firstdevice 102 to the second device 104 are eligible for retransmissionwhile data transmitted in the opposite direction from the second device104 to the first device 102 are not protected by retransmission.However, in other embodiments, the communication system 100 may providefor retransmission in both directions.

For providing retransmission, data are framed prior to the transmissionin basic units which are hereinafter referred to as data transfer units(DTU). The data transfer units may in some embodiments have acontainer-like structure in which integer multiples of higher layer dataunit types such as ATM cells or predefined fractions of Ethernet frames(such as 64/64 B encapsulation portions) are included in a single datatransfer unit. The data transfer unit may be framed such that an integermultiple of a FEC (Forward Error Correction) code word fits into onedata transfer unit. In other words, the data transfer unit issynchronized to integer multiple FEC code words. The FEC code word whichmay for example be a RS (Reed Solomon) code word may however inembodiments be generated after framing the data transfer unit. Thecontainer may also in some embodiments include dummy or padding datawhich are provided within the data transfer unit and may beconfigurable. Each data transfer unit may further include a sequenceidentifier which is a unique identification marking or identifier foreach data transfer unit which allows correctly reassemblingretransmitted data with other non-retransmitted data, i.e. with datawhich is included in a correct received data transfer unit and mayfurther include a time stamp which allows a supervision ofretransmission excess delay in a correct received data transfer unit.

FIG. 2 shows one example of a framing structure 200 for data transferunit which may for example be used in DSL systems such as ADSL(Asymmetric Digital Subscriber Line) or VDSL (Very High Data RateDigital Subscriber Line).

The data transfer unit 200 comprises the sequence identifier 202 and atime stamp 204 provided in a first field associated with a first one ofmultiple FEC code words 206 to which the data transfer unit is matchedto and synchronized with. The time stamp 204 may for example be providedby including a sequence number derived from a DMT (discrete multitonemodulation) symbol counter. The time stamp 204 may indicate an expectedor virtual time in which the data transfer unit is expected to betransmitted by the device 102 to the device 104 or to pass a predefinedpoint in the transmission protocol or process flow. However, in otherembodiments the time stamp 204 can be provided in many other ways as isknown for a person skilled in the art. Payload data 210 obtained fromhigher protocol layers is provided in the data transfer unit. Payloaddata may for example include ATM cell data or fractions of Ethernetpackets. Redundancy Information 212 for the FEC code words may later bedetermined when the data of the data transfer unit container is providedto a FEC processing. Padding bytes 208 are provided in the data transferunit. FIG. 2 shows the padding bytes exemplary associated with the firstFEC code word but it is to be understood that the adding of paddingbytes is configurable and may be different in other embodiments.

It is to be noted that the data transfer unit 200 described with respectto FIG. 2 is only of exemplary character and that other framing typesfor a data transfer unit may be provided. For example, according to oneembodiment, a check sum of the data transfer unit may be provided in oneof the FEC code words.

In order to implement the retransmission, the communication channel 108provides a retransmission return channel (RRC) in the direction reverseto the retransmission channel 106. The retransmission return channelallows a transmission of a retransmission return message from the device104 to the device 102. The retransmission return message may inembodiments be provided continuously with each transmitted datacommunication symbol such as each transmitted DMT symbol. As will bedescribed below in more detail, the retransmission return message mayinclude information such as an acknowledgement information, anidentifier information for identifying to which data transfer unit thereceived retransmission return message refers to and other historicinformation related to the receiving of data transfer units in the past.

The retransmission functionality such as the retransmissionfunctionality for a DSL communication system is provided in the lowestprotocol layer which is also referred to as PHY layer (physical layer).Retransmission functionality in the PHY layer typically hasfundamentally differences to retransmission at layers higher than thePHY layer.

FIG. 3 shows a block diagram 300 of an exemplary implementation of theretransmission functionality in a DSL PHY layer for the first device102. To be more specific, the retransmission is provided in the PMS-TCsub-layer (Physical Media Specific-Transmission Convergence layer)between the TPS-TC sub-layer (Transmission ProtocolSpecific-Transmission Convergence layer) and the PMD sub-layer (PhysicalMedia Dependent layer). The TPS-TC sub-layer, PMS-TC sub-layer, and PMDsub-layer are sub-layers of the PHY layer known to a person skilled inthe art and will therefore not be described in more detail here.

The retransmission functionality receives from a TPS-TC sub-layer 302data such as ATM cells or 64/65 encapsulation data. The received data isprovided to a data transfer unit framer 304 providing framing for thereceived data.

The data transfer unit framer 304 outputs data transfer unit frameswhich are provided to a retransmission multiplexer 306. Retransmissionmultiplexer 306 multiplexes data transfer unit frames from the datatransfer unit framer 304 and data transfer unit containers provided froma retransmission queue 308 for retransmission into a common stream. Datatransfer unit containers output from the retransmission multiplexer areprovided to a FEC encoder 310 and are further copied to theretransmission queue 308 storing the data transfer unit containers forproviding later retransmission of same if necessary. It is to be notedthat other functional entities such as a scrambler may be providedbetween the retransmission multiplexer 306 and the FEC encoder 310.

The FEC encoder 310 provides FEC encoding for the received data transferunit containers and adds the redundancy information to each code word.

The FEC encoder 310 outputs FEC code words to a multiplexer 312 whichmultiplexes the FEC code words with retransmission return informationprovided by a RRC stream 314 and Overhead information from an Overheadstream 316 into a common stream. The common stream output from themultiplexer is transferred for further processing to the PMD layerproviding DMT modulation and other lower layer functions.

The retransmission return message provides information from the device104 to the device 102. In one embodiment the retransmission returnmessage includes a first NACK/ACK bit for providing a positive ornegative acknowledgement of the last received data transfer unit and asecond NACK/ACK bit for providing a positive or negative acknowledgementfor the second-to-last received data transfer unit. The acknowledgementfield for the last received and second last received data transfer unitis hereinafter referred to as NACK[0] and NACK[1]. A positiveacknowledgement is provided when the data transfer unit has beenreceived correct (good). A data transfer unit is received correct whenthe receiver is capable to correctly reconstruct the data of thereceived data transfer unit which may also include using the errorprotection provided in the data transmission symbols.

The retransmission return message further includes in this embodimentthe last five least significant bits (Lsbs) of an absolute data transferunit count number of the last received data transfer unit which ishereinafter referred to as Absolute DTUCountLsbs[4:0]. The AbsoluteDTUCountLsbs[4:0] provides an identifier information to identify towhich transmitted data transfer unit the retransmission return messagerefers to.

Furthermore, in this embodiment a historic information fieldConsecutiveGoodDTUs is provided in the return message. The historicinformation field indicates a number of consecutive correct receiveddata transfer units. The value for the historic information is aninteger value and allows values up to a maximum number max_hist.Max_hist may for example be 31 in one embodiment such that the historicfield is in the form of ConsecutiveGoodDTU [4:0]. Based on whether thesecond-to-last received data transfer unit was correct or corrupt, thehistoric information is determined in different ways. This will beexplained with respect to FIG. 4. As can be seen from FIG. 4, in adecision 402, if the second-to-last data transfer unit was not corrupt,i.e. was received correct, the historic information is determined bycounting the consecutive correct data transfer units prior to the secondto last data transfer unit. If the second to last data transfer unit isa corrupt data transfer unit, a count start data transfer unit ST_1 isdetermined to be the data transfer unit which is Ib data transfer unitsprior to the second-to-last data transfer unit. The parameter Ib whichmay be also referred to as look back parameter is a configurable integernumber. Embodiments for configuring the parameter Ib will be describedin the following.

In embodiments, the value for the parameter Ib is configured based ondetermined REIN properties. To be more specific, the parameter Ib isdetermined based on a determined duration length of REIN impulses andthe determined period of REIN impulses which is the time between thestart of two consecutive REIN impulses. In embodiments, INP_min_reinwhich is a measure of the duration length of a single REIN impulse inunits of corrupt DMT symbols due to the single REIN impulse andia_rein_flag which is a measure for the period of REIN impulses is used.

The value of ia_rein_flag being set to 0 indicates that the REINfrequency is 100 Hz and the duration for a single REIN impulse iat_reinis equal to 1/100 sec=10 msec=10*fs DMT symbols, where fs is the symbolrate in kilo symbols/sec without a unit. The value of iat_rein set to 1indicates that the REIN frequency is equal to 120 Hz and the REINimpulse duration iat_rein is equal to 1/120 sec=25/3 msec*25/3*fs DMTsymbols, where fs is again the symbol rate in kilo symbols/sec without aunit.

In the following embodiments, the value to be configured for Ib iscalculated such that the consecutive correct data transfer unit historyis not obscured by the second-to-last REIN impulse. This will beexplained in more detail with respect to FIG. 5.

FIG. 5 shows an exemplary data transfer unit receive sequence 500 whichis obstructed by two consecutive REIN impulses 502 and 504. FIG. 5further shows a SYNC symbol 506 transmitted between two consecutive datatransfer units of the data transfer unit receive sequence. A SYNC symbolis a predetermined symbol which is transmitted in regular periods forexample after 68 transmitted DMT symbols for providing synchronizationbetween the two devices 102 and 104.

In FIG. 5, the data transfer unit with sequence number n−1 is shown tobe the latest data transfer unit of the sequence which is not obscuredby the second REIN impulse 504. For the calculation of a configurationvalue for the Ib parameter, the requirement is established that thecount start ST_1 (which is in FIG. 5 the data transfer unit withsequence number n-Ib) for the calculation according to 408 in FIG. 4 isnot obscured by the REIN impulse 502.

In mathematical terms, this requirement can expressed by:

Ib=floor((c_impulse_periode−c_impulse_length−a)/(c_DTU_length))−b).

In the above relation, c_impulse_periode is a measure of the period ofthe repetitive impulse noise such as the INP_min_rein value,c_impulse_length is a measure of the length of the repetitive impulsenoise and c_DTU_length is a measure of the length of the data transferunit. Furthermore, a is an integer number equal to 1 which takes intoaccount that a SYNC symbol may be inserted between the two REIN impulses502 and 504. In addition, b is an integer number providing a number ofdata transfer units as a safety measure.

Taking further into account that the historic informationConsecutiveGoodDTUs has a maximum value of hist_max and that the REINfrequency can have 100 Hz (ia_rein_flag=0) or 120 Hz (ia_rein_flag=1),the configuration requirement is obtained by:

ia_rein_flag=0:Ib=min(hist_max;floor((10sec*f−INP_min_rein−a)/(S1*Q))−b),

ia_rein_flag=1:Ib=min(hist_max;floor((25/3sec*fs−INP_min_rein−a)/(S1*Q))−b),

with S1*Q being the data transfer unit size in DMT symbols, INP_min_reinbeing the impulse noise duration in DMT symbols and f being the DMTsymbol rate.

In the following embodiments, the value to be configured for Ib iscalculated such that the consecutive correct data transfer unit historyis not obscured by the second-to-last REIN impulse. This will beexplained in more detail with respect to FIG. 5.

FIG. 5 shows an exemplary data transfer unit receive sequence 500 whichis obstructed by two consecutive REIN impulses 502 and 504. FIG. 5further shows a SYNC symbol 506 transmitted between two consecutive datatransfer units of the data transfer unit receive sequence. A SYNC symbolis a predetermined symbol which is transmitted in regular periods forexample after 68 transmitted DMT symbols for providing synchronizationbetween the two devices 102 and 104.

In FIG. 5, the data transfer unit with sequence number n−1 is shown tobe the latest data transfer unit of the sequence which is not obscuredby the second REIN impulse 504. For the calculation of a configurationvalue for the Ib parameter, the requirement is established that thecount start ST_1 (which is in FIG. 5 the data transfer unit withsequence number n-Ib) for the calculation according to 408 in FIG. 4 isnot obscured by the REIN impulse 502.

In mathematical terms, this requirement can expressed by:

Ib=floor((c_impulse_periode−c_impulse_length−a)/(c_DTU_length))−b).

In the above relation, c_impulse_periode is a measure of the period ofthe repetitive impulse noise such as the INP_min_rein value,c_impulse_length is a measure of the length of the repetitive impulsenoise and c_DTU_length is a measure of the length of the data transferunit. Furthermore, a is an integer number equal to 1 which takes intoaccount that a SYNC symbol may be inserted between the two REIN impulses502 and 504. In addition, b is an integer number providing a number ofdata transfer units as a safety measure.

Taking further into account that the historic informationConsecutiveGoodDTUs has a maximum value of hist_max and that the REINfrequency can have 100 Hz (ia_rein_flag=0) or 120 Hz (ia_rein_flag=1),the configuration requirement is obtained by:

ia_rein_flag=0: Ib=min(hist_max;floor((10sec*fs−INP_min_rein−a)/(S1*Q))−b),

ia_rein_flag=1: Ib=min(hist_max;floor((25/3sec*fs−INP_min_rein−a)/(S1*Q))−b),

with S1′Q being the data transfer unit size in DMT symbols, INP_min_reinbeing the impulse noise duration in DMT symbols and f being the DMTsymbol rate.

In a further embodiment, the calculation of the Ib parameter may bebased on the actual length of the retransmission queue provided indevice 102. Assuming that the length of the data transfer unit includesc data transfer units, a data transfer unit that has been transmittedmore than c data transfer units before the latest retransmission returnmessage is received is no more available at the retransmission queue ofdevice 102. Therefore, any information in the retransmission returnmessage on data transfer units that is older than c data transfer unitsis no more useful because the respective data transfer unit can not beretransmitted anyway.

On the other hand, the larger Ib is provided, the higher is theprobability that the history information can be obscured by previousimpulse noise that is different that the periodic REIN noise. Suchsituation occur for example in an environment of combined REIN and SHINEnoise. In view of this, in one embodiment, the Ib parameter isdetermined such that same is not greater than c where c is theretransmission queue length at the device 102, i.e. at theretransmission transmitter.

In one embodiment, when a data transfer unit transmitted from device 102to device 104 is not acknowledged by a retransmission return messagesend from device 104 to 102, the device 102 cyclic retransmits thisnon-acknowledged data transfer unit every Qth transmitted data transferunit, i.e. after Q−1 other data transfer unit have been transmittedafter the last transmission of this data transfer unit. The cyclicretransmission will be described later in more detail with respect toFIGS. 8a and 8b . For such retransmission scheme, a reference statemachine may be used to determined Qtx which provides best performance.

In other embodiments, the above cyclic transmission is combined with afinal transmission. In this embodiment, when the time period since thevery first transmission of this data transfer unit exceeds apredetermined limit, the data transfer unit is transmitted once more andis then no longer eligible for retransmission. The cyclic retransmissioncombined with a final transmission will be later described in moredetail with respect to FIGS. 8c and 8d . In such a scheme with finalretransmission, the value of Qtx can be choosen much lower and may bedetermined only based on the maximal roundtrip time (maximal roundtripdelay). The round-trip time is the time it takes to process and transmitthe data transfer unit from device 102 to device 104, to receive thedata transfer unit at the device 104 and to generate the retransmissionreturn message for this data transfer unit, to transmit theretransmission return message from the device 104 to the device 102 andto receive this retransmission return and process the retransmissionreturn message at the device 102. In one embodiment, the Ib parametermay therefore be provided by:

Ib=min(hist_max,c),

with hist_max and c as described above. The length c of the queue can inone embodiment take thereby the values of:

c=Qtx (in units of DTUs), in case the reference state machine is used,and

c=maximal roundtrip delay+1 (in units of DTUs).

In one embodiment, the calculation of the value for the Ib parameter maybe based on the periodic impulse noise parameters (REIN period and REINlength) and also be based on the length c of the retransmission queue.In this embodiment, the value for Ib is obtained by:

Ib=min(hist_max;c;floor((c_impulse_periode−c_impulse_length−a)/(c_DTU_length))−b)).

Depending in which region the operator is located, the REIN frequenciesare either 100 Hz (ia_rein_flag=0) and 120 Hz(ia_rein_flag=1). For thesetwo cases, the above equation results in:

ia_rein_flag=0: Ib=min(hist_max;c;floor((10sec*fs−INP_min_rein−a)/(S1*Q))−b),

ia_rein_flag=1: Ib=min(hist_max;c;floor((25/3sec*fs−INP_min_rein−a)/(S1Q))−b).

As described above, in embodiments the value of c may be c=Qtx for casesin which only a cyclic retransmission is used and c=maximal roundtripdelay +1 in cases in which a cyclic retransmission combined with a finalretransmission is used.

In the above described embodiments, the calculation of the value for theIb parameter may be provided either by device 102 or 104 and may betransferred between the two devices.

Having now described the configuration of the parameter Ib, in thefollowing the generating of the retransmission return message during astarting of the communication system in user data mode (showtime) willbe described.

After the initialization of the communication system is completed, bothcommunication channels 106 and 108 are typically not entering showtimeat the same time.

In case the channel 108 enters into showtime later than the channel 106,the device will not receive any meaningful data from the retransmissionreturn channel. In this case, the retransmission transmitter of device102 will not execute any retransmission until the communication channel108 has also entered into showtime.

However, in the case the channel 108 enters into showtime prior to thechannel 106, since the retransmission return message is providedcontinuously, the device 104 has to generate the retransmission returnmessage and to transmit this message to device 102. In the following,different cases or modes can be distinguished. The generating of theretransmission return message in all these cases is based on a virtualextension into the past in which it is assumed that a predeterminednumber N_max_virt of consecutive correct received data transfer unitshas been received in the past by device 104. N_max_virt may in oneembodiment correspond to the maximum value of ConsecutiveGoodDTUs+2. Inone embodiment, in which the maximum value of ConsecutiveGoodDTUs is 31,N_max_virt is obtained to be 33.

Furthermore, it is assumed in the embodiments that a wrap-around of theabsolute data transfer unit counter value occurs at the showtime entryof communication channel 106 i.e. with the first data transfer unittransmitted from device 102 to device 104.

In embodiments, the device 104 will provide in case that no completedata transfer unit has been received from device 102 a retransmissionreturn message which will include only predefined values for theinformation of the message. In such a mode of generating theretransmission return message based on predefined information, thevalues provided in the message correspond to a virtual situation inwhich it is assumed that in the past only correct data transfer unitshave been received. The values in this case provided for the returnmessage are then:

Nack[1:0]=00 which indicates that the last two “virtual received datatransfer units have been correctly received;AbsoluteDTUCountLsbs[4:0]=11111 which indicates that the absolutecounter value provided in the retransmission return message (five leastsignificant bits of the absolute data transfer unit counter value) havea maximum value and therefore the five significant bits of the counterare just before a wrap-around;ConsecutiveGoodDTUs [4:0]=11111 which indicates that the maximum numberfor the historic information of consecutive correct received datatransfer units has been “virtual” received in the past.

The retransmission return message is in embodiments provided as codeword(RRC codeword). The redundancy field is generated based on the providedpredetermined information.

The above described retransmission return message is provided as long asno data transfer unit is received by device 104 from device 102.

In case less than two, i.e. only one data transfer unit is received bythe device 104, a second mode of generating the retransmission returnmessage is provided. Here, at least one of the information, i.e. theacknowledgement of the last received data transfer unit is providedbased on the factual receiving of data transfer unit by device 104.Furthermore, the counter value provided in the retransmission returnmessage is set to the minimum which indicates that the wrap-around inthe absolute counter value has occurred with the beginning of showtimefor communication channel 106. The information provided in this casetherefore are:

Nack[1:0]=0x where x is according to the last factual received datatransfer unit;

AbsoluteDTUCountLsbs[4:0]=00000; ConsecutiveGoodDTUs [4:0]=11111.

In case that more than one but less than a predetermined number of datatransfer units have been received by device 104, the generating of theretransmission return message is provided based on the factual receiveddata transfer unit and the assumption of virtual received data transferunits prior to the first factual receiving of a data transfer unit bydevice 104.

In this mode, the retransmission return message is obtained by:

Nack[1:0]=yx where x is according to the last factual received datatransfer unit and y is according to the second-to-last factual receiveddata transfer unit.

ConsecutiveGood DTUs[4:0]:

If Nack[1]=0, i.e. the second to the last factual received data transferunit is correct, then ConsecutiveGoodDTUs[4:0]=number of consecutivecorrect data transfer units preceding the data transfer unit with countnumber AbsoluteDTUCount-1, wherein the counting includes factualreceived as well as “virtual” received data transfer units;

If Nack[1]=1, i.e. the second to the last factual received data transferunit is corrupt, then ConsecutiveGoodDTUs[4:0]=number of consecutivecorrect data transfer units from and including the data transfer unitwith count number AbsoluteDTUCount-1-Ib, wherein the counting includesfactual received as well as “virtual” received data transfer units.

AbsoluteDTUCountLsbs[4:0]=z where z is the actual absolute count numbercounted from the first received data transfer unit.

The above mode which mixes factual and “virtual” received data transferunits for generating the retransmission return message is provided for apredetermined number of retransmission return messages until no virtualdata transfer units are any longer used for providing the historicinformation ConsecutiveGoodDTUs [4:0].

In the above embodiment, the device 104 may determine based on thenumber of factual received data transfer units in which mode theretransmission return message is to be provided.

It is to be noted that with the above concept of using virtual receiveddata transfer units, an unrequested retransmission is avoided in thecase that the retransmission channel 106 has not or only recentlyentered showtime while channel 108 transmitting the retransmissionreturn messages from device 104 to device 102 is already in showtime.

A further embodiment for providing retransmission return messages fromdevice 104 to the device 102 when communication channel 106 enters intoshowtime is described in the following.

This further embodiment is based on the generation of redundancyinformation for the retransmission return message. In case thecommunication channel 106 has not entered showtime while thecommunication channel 108 is already in showtime, the device 104 will inthis embodiment intentionally manipulate and destroy the redundancyinformation. Device 102 receiving the retransmission return message willtherefore interpret the received retransmission return message as not avalid information. Device 104 will therefore not exert unrequestedretransmissions when communication channel 106 enters showtime.

In another embodiment, the device 104 may generate and transmit theretransmission return messages with correct redundancy information butdevice 102 receiving the return messages may intentionally ignore ordiscard the retransmission return messages received from device 104.This discarding of received retransmission return messages may beprovided for a configurable predetermined time which includes the timein which the communication channel 106 is not in showtime as well as thetime afterwards until the history information of the retransmissionreturn messages can be generated by device 104 fully based on factualreceived data transfer units.

For the generating of the retransmission return message two differentmodes may be used in order to avoid loss of information due to theinsertion of the synchronization DMT symbol as described above.

In such embodiments, a first mode is a strict direct mode in which thedevice 104 inserts the absolute sequence identifier of the most recentreceived data transfer unit into the retransmission return message whengenerating this message. Such a mode alone however causes problems inview of the transmitting of synchronization symbols as will be explainedwith respect to FIG. 6 a.

FIG. 6a shows an embodiment in which a data transfer unit is determinedto have a size of 0.5 transmission symbols, i.e. DMT symbols. The sizeof data transfer unit may be configurable in a certain range for exampleto include from 0.5 up to 4 DMT symbols. For the setting of 0.5, notmore than 2 data transfer units per transmission symbol can be providedwhen transmitting the data transfer unit over the communication channel106. Furthermore, with the retransmission return message having twoacknowledgements fields per RRC code word, which is transmitted everyDMT symbol over the communication channel 108, it could be assumed thata direct acknowledgement for each received data transfer unit isobtained. However, this is not the case due to the transmitting of theDMT sync symbols (for example every 69^(th) symbol) as will be explainedwith respect to FIG. 6 a.

FIG. 6a shows a sequence of DMT symbols labeled “RTX Receiver DMTsymbols” which is received by device 104 (RTX receiver) from device 102(RTX transmitter). The corresponding sequence with the absolute DTUcount numbers of the data transfer units provided in the DMT symbols isshown with label “RTX Receiver Absolute DTU Count of received DTUs”.

FIG. 6a further shows the sequence of DMT symbols transmitted in theother direction from the device 104 to the device 102. This sequence islabeled “RRC Transmitter DMT symbols” in FIG. 6a . The sequence of thecorresponding acknowledgement bits ACK/NACK of the RRC codeword providedin a respective DMT symbol is shown with label “RRC Codeword direct DTUACK/NACK reporting”.

Having now the scheme of the first mode in which the absolute countnumber of the last received and the absolute count number of the secondto last received data transfer unit is acknowledged, it can be seen inFIG. 6a that a correct direct acknowledgement is provided for receivedDTU #2 and DTU #3 and received DTU #4 and DTU #5. However with theinserting of a DMT sync symbol 600 b in the sequence transmitted fromdevice 104 to 102, no codeword can be transmitted during this time slot.Therefore, for the next available DMT data symbol for transmitting theretransmission return message, which is marked in FIG. 6a with 602 b,the last received data transfer unit is the DTU #9 and the second lastreceived data transfer unit is the DTU #8. Strictly applying now thescheme of the first mode, an acknowledgement would result in a gap suchthat no acknowledgement for the received data transfer units DTU #6 and#7 is transmitted from device 104 to device 102. On the other hand, whena DMT sync symbol 600 a is transmitted over channel 106, the datatransfer units received in the DMT symbol just before the DMT syncsymbol would be transmitted twice as shown in FIG. 6 a.

These problems are avoided by generating the acknowledgement flag of theretransmission return message in two modes. In the first mode, asdescribed above, the last and the second to last data transfer unit atthe moment of generating the retransmission return message isacknowledged. In the second mode, the generating of the retransmissionmessage is based on including the absolute count number of the datatransfer units which are included in the second to last DMT symbol andnot the absolute count number of the data transfer units which areincluded in the last DMT symbol.

For the embodiment of having as minimum 2 data transfer units in one DMTsymbol, this can be expressed by the rule:

if AbsoluteDTUCount(x)−AbsoluteDTUCountReport(x−1)>2 then

AbsoluteDTUCountReport(x)=AbsoluteDTUCountReport(x−1)+2

else

AbsoluteDTUCountReport(x)=AbsoluteDTUCount(x)

end.

In the above rule,

AbsoluteDTUCount(x) is the current last received DTU value for RRCcodeword of symbol x in direction from device 104 to device 102,

AbsoluteDTUCountReport(x−1) is the Absolute DTU Count value forreporting in the last RRC codeword, and

AbsoluteDTUCountReport(x) is the Absolute DTU Count value for reportingin the current RRC codeword.

Furthermore, AbsoluteDTUCountLsbs[4:0] is provided to be the 5 lastsignificant bits of the AbsoluteDTUCountReport as determined by theabove rule.

With this scheme using two different modes for providing theacknowledgement information, the device 104 executes the DTU basedreporting in such a way that no direct gaps in the acknowledgementinformation (NACK/ACK) appear.

The device 104 executes the strict last received DTU based reportingonly if no direct ACK/NACK reporting gaps happen. If these gaps wouldhappen according to the strict last received reporting, the currentreporting is switched to a second mode in which the reporting is basedon the second data transfer unit after the last received data transferunit of the previous RRC codeword.

FIG. 6b shows the sequences of FIG. 6b with the two-mode-reporting. Itcan be seen that no gap appears in the RRC codeword sequence which istransmitted from device 104 to device 102.

Based on the receiving of a retransmission return message, the device102 may provide a retransmission of one or more data transfer unitsstored in the retransmission buffer.

In the case that the device 102 receives no acknowledgement or anegative acknowledgement of the transmitted data transfer unit forexample due to a long SHINE impulse disturbing the retransmission returnchannel, the following scheme is applied. The data transfer unit in theretransmission queue is retransmitted exactly after a predeterminednumber (Qtx−1) of other data transfer units that have been transmittedby the device 102 since the last transmission of this data transfer unitif no acknowledgement has been received for the data transfer unit untilthen. In other words, any unacknowledged data transfer unit at the endof the retransmission queue is retransmitted exactly every Qtx datatransfer units. An upper time limit herein after referred to asdelay_max is defined for the above described retransmitting ofunacknowledged data transfer units. Delay_max defines for each datatransfer unit a predetermined time limit or threshold. Once this timelimit has exceeded by the data transfer unit, the data transfer unit isno longer eligible for retransmission and will not be transmitted.

In addition to the above described cyclic repetition every Qtx datatransfer units, the retransmission scheme includes a final-transmissionscheme which may also be referred to as last-chance-retransmission.Whenever a data transfer unit that is anywhere in the retransmissionqueue exceeds a predetermined final-transmission time limit or thresholdwhich is provided to be before the end of delay_max, the data transferunit will be transmitted. In other words, the providing of the finaltransmission is independent of the position of this data transfer unitin the queue and is based only on the final-transmission time limit. Inone embodiment, the final-transmission time limit is configured to bewithin the last transmission which is possible before delay_max isexceeded. Thus, in this embodiment, if it is determined that any datatransfer unit would exceed the delay_max constraint if it would beretransmitted later than the next outgoing data transfer container, thisdata transfer unit is retransmitted with the next possible outgoing datatransfer container. In another embodiment, the final transmission timelimit may be configured to be one or more data transfer units ahead ofthe last possible transmission which may add more safety that the datatransfer unit has not exceeded the delay_max when it is scheduled fortransmission.

FIG. 7 shows a flow chart 700 according to an embodiment of the combinedcyclic retransmission and final transmission.

In 702, a transmission of a previously transmitted data transfer unit isrepeated after a predetermined number of other data transfer units hasbeen transmitted. In 704, it is determined whether a measure for a timeperiod since the first transmission of the data transfer unit hasexceeded a predetermined threshold. In other words, it is determined in704 whether a measure for the time which has passed since the very firsttransmission of the previously transmitted data transfer unit and theactual time when making the decision exceeds a limit or threshold. Thedetermining may for example be implemented by using a time stamp of thepreviously transmitted for comparing with a threshold. In anotherembodiment, it may be determined whether a predefined number of datatransfer units have been transmitted since the very first transmissionof this previously transmitted data transfer unit. However, in otherembodiments other ways of making the determining whether the time periodbetween the very first transmitting and the actual time when making thedetermination exceeds the limit or threshold may be provided. In 706, afinal transmission of the previously transmitted data transfer unit isprovided based on the determining that the measure for the time periodhas exceeded the predetermined threshold.

In embodiments, after the final-transmission is provided for a datatransfer unit, a marking is provided which induces that this data unitis no longer eligible for retransmission. In one embodiments, the datatransfer unit is marked as acknowledged which induces that no furthertransmission will be provided by device 102. No other changes are donein the buffer, and the transmitted data transfer unit will not be fedinto the beginning of the queue.

It is to be noted that this final transmission will be provided for adata transfer unit even if the time period before the previoustransmission of this data transfer unit is so short that anacknowledgement could not be received in view of the round-trip time ofthe communication system. The round-trip time is a measure for the timeit takes to process and transmit the data transfer unit at the device102, to receive and process the data transfer unit at the device 14, toprocess and transfer the retransmission return message at the device 104and to receive and process the retransmission return message at thedevice 102.

Since the final transmission can be regarded as some kind of preemption,the transmission of a data transfer unit which is scheduled fortransmission according to the above described cyclic scheme has to bedelayed by one data transmission unit container.

The final transmission scheme provides a better performance in caseswhen long noise impulses such as long SHINE impulses occur. This can beseen with respect to FIGS. 8a to 8 d.

FIG. 8a shows a data transfer unit 800 which is obscured by a SHINEimpulse 802 a. The SHINE impulse 802 a does not allow receivingacknowledgements for the transmitted data transfer units. The shineimpulse 802 a is in FIG. 8a short enough such that the cyclic scheme oftransmitting every Qtx data transfer units alone succeeds inretransmitting all data transfer units after the SHINE impulse 802 a hasended before delay_max has ended. Here the cyclic scheme alone iscapable of successfully providing retransmission for all data transferunits without the need for a final transmission.

It becomes clear that the cyclic scheme alone is successful when:

ceil(IL_SHINE/Qtx)*Qtx<delay_max*fs/(S1*Q),

wherein IL_SHINE is the amount of corrupted data transfer units as seenat the delta interface of the physical layer and Qtx is thepredetermined number of data transfer units, delay_max is the maximumallowed retransmission period in msec, fs is the symbol rate in kilosymbols/sec and S1Q is the size of a data transfer units in symbols.

If the above condition is not met, the cyclic scheme alone may not allowthe retransmission of all data transfer units before the end ofdelay_max.

Such a situation is shown in FIG. 8b . FIG. 8b shows a situation for alonger SHINE impulse 802 b when only the cyclic scheme would be applied.It can be seen that the data transfer unit with sequence identifiernumber (SID) 17 which is the first data transfer unit obscured by theSHINE impulse 802 b would not be transmitted successful because, afterthe ending of SHINE impulse 802 b, the data transfer unit with SID 17 isscheduled by the cyclic scheme to be transmitted after delay_max. It isto be noted that not only the data transfer unit with SID 17 but alsothe data transfer unit with SID 18 would not be transmitted by thecyclic scheme alone.

FIG. 8c shows the same situation for the cyclic scheme including thefinal transmission scheme. In FIG. 8c , the data transfer unit with SID17 is included in the last possible container before delay_max ends forthis data transfer unit. Thus, with the final transmission scheme,instead of transmitting in this container the data transfer unit withSID 20 (see FIG. 8b ), the final transmission data transfer unit withSID 17 is transmitted due to the final transmission of this datatransfer unit. Furthermore, for the next container, instead oftransmitting in this container the data transfer unit with SID 21 (seeFIG. 8b ), the data transfer unit with SID 18 is transmitted due to thefinal transmission of this data transfer unit. For the followingcontainer, instead of transmitting in this container the data transferunit with SID 22 (see FIG. 8b ), the data transfer unit with SID 19 istransmitted due to the final transmission of this data transfer unit. Itis to be noted that although the data transfer unit with SID 19 has beentransmitted right after the ending of the SHINE impulse and therefore islikely to be received correct, the final transmission for this datatransfer unit is provided because no acknowledgement has been receiveddue to the round-trip time until the point in time when it is decided toprovide a final transmission for this data transfer unit.

FIG. 8d shows for the data transfer unit with SID 17 the retransmissionprovided. It can be seen that until the final transmission is provided,three retransmissions have been performed in the cyclic scheme, thedistance between all being constant. The final transmission is thenperformed in a non-regular manner three data transfer units after thelast transmission of this data transfer unit.

It becomes clear that with the including of a final-transmission schemein the cyclic scheme, an improved protection performance is achieved forthe communication system. For the minimum Impulse Noise protectionprovided by the above described scheme including a final transmission,the following constraint can be established:

INP_min≤floor(floor delay_max*fs)(S*Q))*(S*Q).

FIG. 9 shows an exemplary flow diagram 900 according to an embodiment ofthe present invention.

The flow diagram determines at 902 whether a time period since the firsttransmission of a data transfer unit has exceeded a predetermined timelimit. If it is determined at 902 that the predetermined time limit hasbeen exceeded, then the final transmission is provided at 904 and theprocess ends. If it is determined at 902 that the time period has notexceeded, then process proceeds to 906 where it is determined whether(Qtx−1) data transfer units have been transmitted since the lasttransmission of this data transfer unit. If it is determined at 906 that(Qtx−1) data transfer units have not been transmitted, the processreturns to 902. If it is determined at 906 that (Qtx−1) data transferunits have been transmitted, then the process proceeds to 908 where itis determined whether a negative or no acknowledgement has beenreceived. If it is decided that a negative or no acknowledgement has notbeen received, i.e. a positive acknowledgement has been received, theprocess ends. If it is determined at 908 that no acknowledgment or anegative has been received, the data transfer unit is transmitted at 910and the process returns to 902.

In the above description, embodiments have been shown and describedherein enabling those skilled in the art in sufficient detail topractice the teachings disclosed herein. Other embodiments may beutilized and derived there from, such that structural and logicalsubstitutions and changes may be made without departing from the scopeof this disclosure.

This Detailed Description, therefore, is not to be taken in a limitingsense, and the scope of various embodiments is defined only by theappended claims, along with the full range of equivalents to which suchclaims are entitled.

Such embodiments of the inventive subject matter may be referred toherein, individually and/or collectively, by the term “invention” merelyfor convenience and without intending to voluntarily limit the scope ofthis application to any single invention or inventive concept if morethan one is in fact disclosed. Thus, although specific embodiments havebeen illustrated and described herein, it should be appreciated that anyarrangement calculated to achieve the same purpose may be substitutedfor the specific embodiments shown. This disclosure is intended to coverany and all adaptations or variations of various embodiments.Combinations of the above embodiments, and other embodiments notspecifically described herein, will be apparent to those of skill in theart upon reviewing the above description.

It is further to be noted that specific terms used in the descriptionand claims may be interpreted in a very broad sense. For example, theterms “circuit” or “circuitry” used herein are to be interpreted in asense not only including hardware but also software, firmware or anycombinations thereof. The term “data” may be interpreted to include anyform of representation such as an analog signal representation, adigital signal representation, a modulation onto carrier signals etc.The term “information” may in addition to any form of digitalinformation also include other forms of representing information. Theterm “entity” or “unit” may in embodiments include any device, apparatuscircuits, hardware, software, firmware, chips or other semiconductors aswell as logical units or physical implementations of protocol layersetc. Furthermore the terms “coupled” or “connected” may be interpretedin a broad sense not only covering direct but also indirect coupling.

It is further to be noted that embodiments described in combination withspecific entities may in addition to an implementation in these entityalso include one or more implementations in one or more sub-entities orsub-divisions of the described entity. For example, specific embodimentsdescribed herein described herein to be implemented in a transmitter,receiver or transceiver may be implemented in sub-entities such as achip or a circuit provided in such an entity.

The accompanying drawings that form a part hereof show by way ofillustration, and not of limitation, specific embodiments in which thesubject matter may be practiced.

In the foregoing Detailed Description, it can be seen that variousfeatures are grouped together in a single embodiment for the purpose ofstreamlining the disclosure. This method of disclosure is not to beinterpreted as reflecting an intention that the claimed embodimentsrequire more features than are expressly recited in each claim. Rather,as the following claims reflect, inventive subject matter lies in lessthan all features of a single disclosed embodiment. Thus the followingclaims are hereby incorporated into the Detailed Description, where eachclaim may stand on its own as a separate embodiment. While each claimmay stand on its own as a separate embodiment, it is to be notedthat—although a dependent claim may refer in the claims to a specificcombination with one or more other claims—other embodiments may alsoinclude a combination of the dependent claim with the subject matter ofeach other dependent claim. Such combinations are proposed herein unlessit is stated that a specific combination is not intended.

It is further to be noted that methods disclosed in the specification orin the claims may be implemented by a device having means for performingeach of the respective steps of these methods.

1-26. (canceled)
 27. A digital subscriber line (DSL) noise protectionapparatus, the apparatus comprising: first circuitry to receive datatransfer units from a downstream direction including at least one datatransfer unit corrupted by noise; second circuitry to obtain aretransmission return message with a number of the data transfer unitsacknowledged; the second circuitry to determine a start of the datatransfer units acknowledged based on a look back value; and the secondcircuitry to communicate the look back value in an upstream direction.28. The apparatus of claim 27, the second circuitry to generate the lookback value during showtime.
 29. The apparatus of claim 27, the secondcircuitry to generate the look back value.
 30. The apparatus of claim29, the second circuitry to generate the retransmission return message.31. The apparatus of claim 27, wherein the look back value is based on alength of a retransmission queue.
 32. The apparatus of claim 27, whereinthe second circuitry is to obtain the number from a non-acknowledgmentsignal field in the retransmission return message.
 33. The apparatus ofclaim 27, further comprising a framer.
 34. The apparatus of claim 27,further comprising a retransmission queue.
 35. The apparatus of claim27, further comprising a multiplexer.
 36. The apparatus of claim 27,wherein the second circuitry is further to determine when one of thedata transfer units is to pass a predefined point in the transmissionprotocol based on a timestamp.
 37. The apparatus of claim 27, whereinthe first circuitry is to receive the at least one data transfer unitcorrupted by noise selected from the group consisting of impulse noise,repetitive impulse noise, single high impulse noise and long noise. 38.A digital subscriber line (DSL) noise protection apparatus, theapparatus comprising: first circuitry to transmit data transfer units ina downstream direction; second circuitry to obtain a retransmissionreturn message indicating a number of acknowledged data transfer units;wherein the second circuitry further to retransmit data transfer unitsnot acknowledged based on a look back value indicating a start of theacknowledged data transfer units; and wherein the second circuitryfurther to receive the look back value from an upstream direction. 39.The apparatus of claim 38, the second circuitry to obtain the look backvalue during showtime.
 40. The apparatus of claim 38, the secondcircuitry further to receive the retransmission return message from theupstream direction.
 41. The apparatus of claim 38, wherein the look backvalue is based on a length of a retransmission queue.
 42. The apparatusof claim 38, wherein the second circuitry is to obtain the number from anon-acknowledgment signal field in the retransmission return message.43. The apparatus of claim 38, wherein the second circuitry includes aframer.
 44. The apparatus of claim 38, wherein the second circuitryincludes a retransmission queue.
 45. The apparatus of claim 38, whereinthe second circuitry includes a multiplexer.
 46. The apparatus of claim38, wherein the first circuitry is further to determine when one of thedata transfer units is to pass a predefined point in the transmissionprotocol based on a timestamp.
 47. The apparatus of claim 38, whereinthe first circuitry to transmit data transfer units subject to noiseselected from the group consisting of impulse noise, repetitive impulsenoise, single high impulse noise and long noise.
 48. A digitalsubscriber line (DSL) noise protection apparatus, the apparatuscomprising: receiver circuitry to receive data transfer units from adownstream direction including a data transfer unit corrupted by noise;retransmisson circuitry to obtain a retransmission retum message with anumber of the data transfer units acknowledged; logic circuitry todetermine a start of the data transfer units acknowledged based on alook back value; and wherein the logic circuitry to communicate the lookback value in an upstream direction.
 49. A digital subscriber line (DSL)noise protection system, the system comprising: a first communicationdevice to prepare data transfer units to be transmitted; a secondcommunication device to receive the data transfer units transmittedincluding at least one data transfer unit corrupted by noise; whereinthe second communication device to obtain a retransmission returnmessage with a number of the data transfer units acknowledged; whereinthe second communication device to determine a start of the datatransfer units acknowledged based on a look back value; and wherein thesecond communication device is to communicate the look back value to thefirst communication device.
 50. The system of claim 49, wherein thefirst communication device is to retransmit data transfer units notacknowledged based on the look back value.
 51. The system of claim 49,wherein the second communication device is to transmit the look backvalue to the first communication device during showtime.
 52. The systemof claim 49, wherein the second communication device generates theretransmission return message.
 53. A digital subscriber line (DSL) noiseprotection method, the method comprising: receiving data transfer unitsincluding at least one data transfer unit corrupted by noise from adownstream direction; obtaining a retransmission retum message with anumber of the data transfer units acknowledged; determining a start ofthe data transfer units acknowledged based on a look back value; andcommunicating the look back value in an upstream direction.
 54. Themethod of claim 53, further comprising transmitting the look back valuein an upstream direction during showtime.
 55. The method of claim 53,further comprising generating the retransmission return message.
 56. Themethod of claim 53, further receiving the at least one data transferunits corrupted by noise selected from the group consisting of impulsenoise, repetitive impulse noise, single high impulse noise and longnoise.
 57. A digital subscriber line (DSL) noise protection method, themethod comprising: transmitting data transfer units in a downstreamdirection; receiving a retransmission retum message with a number of thedata transfer units acknowledged; determining a start of the datatransfer units acknowledged based on a look back value; and receivingthe look back value from an upstream direction.
 58. The method of claim57, further comprising receiving the look back value in an upstreamdirection during showtime.
 59. The method of claim 57, wherein the datatransfer units transmitted subject to noise selected from the groupconsisting of impulse noise, repetitive impulse noise, single highimpulse noise and long noise.